From 578bc73f12a97eaa3ca4ccc8d212f2a62a29fcbc Mon Sep 17 00:00:00 2001
From: LHMQ878 <72402929@cityu-dg.edu.cn>
Date: Thu, 6 Aug 2026 19:30:22 +0800
Subject: [PATCH] fix(json_schema): keep min/maxLength when format falls back
 to str

_create_string_type returned early for any format, dropping minLength,
maxLength, and pattern even when the format resolves to a plain str
(custom formats / uri-reference).

Fixes #4404
---
 .../fastmcp/utilities/json_schema_type.py     | 24 ++++++++++++-------
 .../json_schema_type/test_formats.py          | 19 +++++++++++++++
 2 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/fastmcp_slim/fastmcp/utilities/json_schema_type.py b/fastmcp_slim/fastmcp/utilities/json_schema_type.py
index de1db2900..b3e64ffb5 100644
--- a/fastmcp_slim/fastmcp/utilities/json_schema_type.py
+++ b/fastmcp_slim/fastmcp/utilities/json_schema_type.py
@@ -269,13 +269,6 @@ def _create_string_type(schema: Mapping[str, Any]) -> type | Annotated[Any, ...]
     if "const" in schema:
         return Literal[schema["const"]]  # type: ignore
 
-    if fmt := schema.get("format"):
-        if fmt == "uri":
-            return AnyUrl
-        elif fmt == "uri-reference":
-            return str
-        return FORMAT_TYPES.get(fmt, str)
-
     constraints = {
         k: v
         for k, v in {
@@ -286,7 +279,22 @@ def _create_string_type(schema: Mapping[str, Any]) -> type | Annotated[Any, ...]
         if v is not None
     }
 
-    if not constraints:
+    # Resolve format first. Non-str formats (date-time, email, uri, json) do not
+    # take length/pattern constraints. Str-backed formats (unknown custom names
+    # and uri-reference) must still honor minLength/maxLength/pattern.
+    if fmt := schema.get("format"):
+        if fmt == "uri":
+            return AnyUrl
+        if fmt == "uri-reference":
+            base: type | Annotated[Any, ...] = str
+        else:
+            base = FORMAT_TYPES.get(fmt, str)
+        if base is not str:
+            return base
+        if not constraints:
+            return str
+        # Fall through to apply StringConstraints on the str-backed format.
+    elif not constraints:
         return str
 
     annotated: Any = Annotated[str, StringConstraints(**constraints)]
diff --git a/tests/utilities/json_schema_type/test_formats.py b/tests/utilities/json_schema_type/test_formats.py
index 781b81a82..b36bafd36 100644
--- a/tests/utilities/json_schema_type/test_formats.py
+++ b/tests/utilities/json_schema_type/test_formats.py
@@ -112,3 +112,22 @@ class TestFormatTypes:
         )
         assert isinstance(result.full_uri, AnyUrl)
         assert isinstance(result.ref_uri, str)
+
+    def test_str_backed_format_keeps_max_length(self):
+        """Unknown/custom formats fall back to str and must still enforce length."""
+        t = json_schema_to_type(
+            {"type": "string", "format": "phone", "maxLength": 3}
+        )
+        validator = TypeAdapter(t)
+        assert validator.validate_python("123") == "123"
+        with pytest.raises(ValidationError):
+            validator.validate_python("abcd")
+
+    def test_uri_reference_format_keeps_min_length(self):
+        t = json_schema_to_type(
+            {"type": "string", "format": "uri-reference", "minLength": 2}
+        )
+        validator = TypeAdapter(t)
+        assert validator.validate_python("/a") == "/a"
+        with pytest.raises(ValidationError):
+            validator.validate_python("")